Anode effluent control in fuel cell power plant

ABSTRACT

A fuel cell stack ( 1 ) performs power generation using an anode gas having hydrogen as its main component, and after a power generation reaction, the anode gas is discharged as anode effluent. The anode effluent is re-circulated into the anode gas through a return passage ( 5 ). The return passage ( 5 ) comprises a purge valve ( 8 ) which discharges the anode effluent to the outside of the passage. In this invention, calculation of a first energy loss caused by an increase in non-hydrogen components in the anode gas while the purge valve ( 8 ) is closed (S 7 , S 28 ), and calculation of a second energy loss which corresponds to the amount of hydrogen lost from the anode gas by opening the purge valve ( 8 ) (S 8 , S 29 ) are performed. By opening the purge valve ( 8 ) when the second energy loss equals or falls below the first energy loss, the start timing of purging is optimized.

FIELD OF THE INVENTION

This invention relates to the removal of impurity gases within anode gasthat is supplied to a fuel cell stack.

BACKGROUND OF THE INVENTION

In a fuel cell power plant comprising a re-circulation passage whichre-circulates anode effluent discharged from an anode into the hydrogenthat is supplied to the anode, impurity gases such as nitrogen becomemixed into the anode gas as an operation of the fuel cell power plantprogresses. Such impurity gases cause a reduction in the hydrogenpartial pressure within the anode gas, thus leading to deterioration inthe output performance of the fuel cell stack.

JP2000-243417A, published by the Japan Patent Office in 2000, disclosesa method of removing impurity gases by purging the gas in there-circulation passage in accordance with a decrease in the hydrogenconcentration of the anode gas or a decrease in the output of the powerplant.

JP2001-006709A, published by the Japan Patent Office in 2001, disclosesa method of preventing poisoning of the anode caused by carbon monoxide(CO) contained in the anode gas by mixing a small amount of oxygen intothe anode gas.

SUMMARY OF THE INVENTION

By purging the gas in the re-circulation passage, however, the originalhydrogen that is required for power generation is purged together withthe impurity gases.

By mixing a small amount of oxygen into the anode gas, not only iscarbon monoxide (CO) removed, but a part of the hydrogen reacts with theoxygen to form water (H₂O), and hence a part of the hydrogen gas to beused in the power generation reaction is lost.

For such reasons, measures taken in any of the prior art to removeimpurity gases may instead lead to a reduction in the power generationefficiency of the fuel cell. In other words, if the purging timing andoxygen mixing timing in the prior art are not set appropriately, theexpected improvement in power generation efficiency cannot be realized.

It is therefore an object of this invention to enable an improvement inthe power generation efficiency of a fuel cell power plant by settingthe timing of such an impurity gas removal operation appropriately.

In order to achieve the above object, this invention provides an anodeeffluent control method for a such a fuel cell power plant thatcomprises a fuel cell stack which performs power generation using anodegas having hydrogen as a main component. In the power plant, the anodegas is discharged from the fuel cell stack as anode effluent following apower generation reaction. The power plant further comprises a returnpassage which re-circulates the anode effluent into the anode gas, and apurge valve which discharges the anode effluent in the return passage tothe outside of the passage.

The control method comprises calculating a first energy loss caused byan increase in a non-hydrogen component in the anode gas while the purgevalve is closed, calculating a second energy loss which corresponds toan amount of hydrogen lost from the anode gas when the purge valve isopened, maintaining the purge valve in a closed state when the secondenergy loss is larger than the first energy loss, and opening the purgevalve when the second energy loss equals or falls below the first energyloss.

This invention also provides an anode effluent control device for thefuel cell stack as described above. The device comprises a returnpassage which re-circulates the anode effluent into the anode gas, apurge valve which discharges the anode effluent in the return passage tothe outside of the passage, and a programmable controller to controlopening and closing of the purge valve. The controller is programmed tocalculate a first energy loss caused by an increase in a non-hydrogencomponent in the anode gas while the purge valve is closed, calculate asecond energy loss which corresponds to an amount of hydrogen lost fromthe anode gas when the purge valve is opened, maintain the purge valvein a closed state when the second energy loss is larger than the firstenergy loss, and open the purge valve when the second energy loss equalsor falls below the first energy loss.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell power plant according tothis invention.

FIG. 2 is a diagram illustrating the relationship between impurities andreductions in the output of a fuel cell stack.

FIG. 3 is a diagram illustrating the relationship between the durationof an operation of the fuel cell stack and the energy balance.

FIG. 4 is a flowchart illustrating a purging start determination routineexecuted by a controller according to this invention.

FIG. 5 is a diagram illustrating the characteristic of a map, which isstored in the controller, of a water vapor partial pressure PWSn in theanode gas.

FIG. 6 is a diagram illustrating the characteristic of a map, which isstored in the controller, defining the relationship between hydrogenpartial pressure PH2n in the anode gas and the generated energy EDH2n ofthe fuel cell stack.

FIG. 7 is a diagram illustrating the characteristic of a map, which isstored in the controller, of variation ΔEDPn in the hydrogen energywhich accumulates in the anode gas in relation to purging not beingperformed.

FIG. 8 is a schematic diagram of a fuel cell power plant according to asecond embodiment of this invention.

FIG. 9 is a flowchart illustrating a purging start determination routineexecuted by a controller according to the second embodiment of thisinvention.

FIG. 10 is a flowchart illustrating a carbon monoxide removaldetermination routine executed by the controller according to the secondembodiment of this invention.

FIG. 11 is a flowchart illustrating a carbon monoxide removal routineexecuted by the controller according to the second embodiment of thisinvention.

FIG. 12 is a diagram illustrating the characteristic of a hydrogensupply amount map which is stored in the controller according to thesecond embodiment of this invention.

FIG. 13 is a diagram illustrating the conversion rate of a catalystaccording to the second embodiment of this invention.

FIG. 14 is a schematic diagram of a fuel cell power plant according to athird embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a fuel cell stack 1 is constitutedby a stacked body of fuel cells which perform power generation by meansof an electrochemical reaction between hydrogen that is supplied to ananode 1A as anode gas and oxygen in the air that is supplied to acathode 1B as cathode gas through a electrolyte membrane 1C.

Hydrogen is supplied to the anode 1A through a hydrogen supply passage 4from a high-pressure hydrogen tank 3. A control valve 6 is provided onthe hydrogen supply passage 4 to control the flow rate of hydrogensupplied to the hydrogen supply passage 4 from the hydrogen tank 3.

Anode effluent that is discharged from the anode 1A after the reactionis re-supplied to the hydrogen supply passage 4 through an ejector 9from a return passage 5.

Air is supplied to the cathode 1B by a compressor 2.

A purge passage 7 for purging the anode effluent outside is connected tothe return passage 5 via a purge valve 8.

As anode effluent continues to be re-circulated into the hydrogen supplypassage 4 from the return passage 5 during an operation of the fuel cellpower plant, impurities in the anode gas that is supplied to the anode1A increase, causing a reduction in the hydrogen partial pressure PH2n,which leads to a reduction in the output of the fuel cell stack 1.

A controller 11 removes the impurities in the anode gas that is suppliedto the anode 1A by controlling the opening of the control valve 6 andpurge valve 8.

The controller 11 is constituted by a microcomputer comprising a centralprocessing unit (CPU), read-only memory (ROM), random access memory(RAM), and an input/output interface (I/O interface). The controllermaybe constituted by a plurality of microcomputers.

A temperature signal from a temperature sensor 10 which detects thetemperature of the fuel cell stack 1 is input into the controller 11 asa parameter for this control.

In addition to hydrogen, the following impurity gases or gas componentsare contained in the anode effluent that is discharged to the returnpassage 5 from the fuel cell stack 1:

-   -   (1) gas having as its main component nitrogen that flows into        the anode 1A from the cathode 1B through the electrolyte        membrane 1C, and    -   (2) gas components such as carbon monoxide (CO) contained in the        hydrogen that is supplied from the hydrogen tank 3.

Re-circulation of the anode effluent causes these impurity gases and gascomponents to mix with the hydrogen in the hydrogen supply passage 4,leading to a reduction in the hydrogen partial pressure PH2n of theanode gas. As a result, as shown in FIG. 2, the output voltage of thefuel cell 1 decreases as specified by ΔV in the figure, and thus thepower generation efficiency of the fuel cell 1 deteriorates.

The amount of impurity gases increases as the fuel cell 1 continues tooperate while re-circulating anode effluent, and the hydrogen partialpressure PH2n gradually decreases accordingly.

Referring to FIG. 3, energy loss caused by a reduction in the hydrogenpartial pressure PH2n increases in relation to the duration of anoperation of the fuel cell 1 in which anode effluent is re-circulated,as shown by the solid line in the diagram. The rate of increase alsoincreases as the duration of the operation increases. The abscissa ofthe diagram indicates the elapsed time tn from the end of purging.

Conversely, when the purge valve 8 is opened to purge the anode effluentin the return passage 5, the impurities in the anode gas that issupplied to the anode 1A decrease, but at the same time, the hydrogencontained in the anode effluent is also purged. If purging is begun whenthe hydrogen concentration in the anode gas reaches a predeterminedconcentration, and purging is halted when another predeterminedconcentration is reached, the hydrogen energy lost by purging isconstant, regardless of the purging duration. The hydrogen energy thatis discharged by purging becomes gradually smaller as the purgingduration shortens.

On the other hand, since the abscissa of FIG. 3 indicates the elapsedtime from the end of purging, if purging is resumed at a timingcorresponding to the elapsed time of tn, then the elapsed time tncorresponds to the purging interval. As shown by the broken line in FIG.3, the hydrogen energy that is lost when purging is not performedbecomes smaller as the purging interval becomes longer. The sum of theenergy loss due not to perform purging and the energy loss due topurging takes a minimum value at a point A, as shown by the dotted linein the diagram. In other words, in the section between the end ofpurging and point A, or in the time period from t0 (=0) to tn, energyloss is smaller when purging is not performed, but when the elapsed timepasses the point A, or after the time tn, energy loss is larger whenpurging is not performed. Hence in terms of energy balance, purging ispreferably performed at the point A.

Next, referring to FIG. 4, a purging start determination routineexecuted by the controller 11 to determine the start of purging will bedescribed. This routine is implemented repeatedly by the controller 11at a fixed interval Δt while the fuel cell power plant is operative. Thefixed interval Δt is set at 100 milliseconds.

First, in a step S1, the controller 11 checks a purge flag fp. The purgeflag fp indicates whether or not anode effluent purging is beingperformed through the purge valve 8. The purge flag fp has a value ofeither unity or zero. The initial value of the purge instruction flag fpis zero.

When the purge flag fp is at unity, this indicates that purging isunderway. In this case, the controller 11 performs the processing in astep S12 to be described below.

When the purge flag fp is at zero, the controller 11 calculates theoperating duration tn from the end of the previous purging operation ina step S2. This calculation is performed by adding the routine executioninterval Δt to a previous routine start time tn⁻¹.

Next, the nitrogen partial pressure PNn of the anode gas is calculatedin a step S3.

The controller 11 performs this calculation using the followingequations (1) and (2).

First, the amount of nitrogen gas permeating the electrolyte membrane 1Cfrom the cathode 1B to the anode 1A of the fuel cell stack 1 from thetime tn⁻¹ to the time tn is calculated according to the followingequation (1).

$\begin{matrix}{{{{Amount}\mspace{14mu}{of}{\;\mspace{11mu}}{permeated}{\mspace{11mu}\;}{nitrogen}{\mspace{11mu}\;}{gas}} = {{K \cdot \frac{{membrane}{\mspace{11mu}\;}{area}}{{membrane}{\mspace{11mu}\;}{thickness}} \cdot \left( {{nitrogen}\mspace{14mu}{partial}\mspace{14mu}{{pressur}e}\mspace{14mu}{difference}\mspace{14mu}{between}\mspace{14mu}{anode}\mspace{14mu}{gas}\mspace{14mu}{and}\mspace{14mu}{cathode}\mspace{14mu}{gas}} \right) \cdot \Delta}\; t\mspace{11mu}{where}}},\mspace{14mu}{K = {{gas}{\mspace{11mu}\;}{permeation}\mspace{14mu}{coefficient}\mspace{14mu}{of}\mspace{14mu}{the}{\mspace{11mu}\;}{electrolyte}\mspace{14mu}{membrane}\mspace{14mu} 1\; C\mspace{14mu}{during}\mspace{14mu}{an}\mspace{14mu}{{operation}.}}}} & (1)\end{matrix}$

When the fuel cell stack 1 is operated in a steady state, the nitrogenpartial pressure of the cathode gas is substantially constant.Conversely, the nitrogen partial pressure PNn of the anode gas graduallyincreases over time due to the nitrogen gas which permeates theelectrolyte membrane 1C. This relationship can be determined byexperiment, simulation, or calculation on the basis of thecharacteristic of the electrolyte membrane 1C.

A map of the nitrogen gas partial pressure difference between the anodeand cathode in accordance with the operating duration tn is stored inadvance in the internal memory (ROM) of the controller 11, and thenitrogen partial pressure difference between the anode gas and cathodegas is determined upon calculation of the equation (1) by referring tothis map on the basis of the operating duration tn. The membrane areaand membrane thickness of the electrolyte membrane 1C are known fixedvalues.

The value of the permeated nitrogen gas amount of the equation (1),having been time-integrated according to the equation (2), is theoverall permeated nitrogen gas amount from an operating duration of zeroto tn.overall permeated nitrogen gas amount=∫(permeated nitrogen gasamount)∝∫(nitrogen gas partial pressure difference between anode andcathode)·Δt  (2)

Accordingly, the nitrogen partial pressure PNn of the anode gas isexpressed by the following equation (3).

$\begin{matrix}{{PNn} = {\left( {{anode}\mspace{14mu}{gas}{\mspace{11mu}\;}{pressure}} \right) \cdot \frac{\begin{matrix}{{normal}\text{-}{pressure}\mspace{14mu}{volume}} \\{{of}\mspace{14mu}{nitrogen}\mspace{14mu}{in}\mspace{14mu}{passage}}\end{matrix}}{\frac{\begin{matrix}{\left( {{passage}{\mspace{11mu}\;}{volume}} \right) \cdot} \\\left( {{anode}{\mspace{11mu}\;}{gas}\mspace{14mu}{pressure}} \right)\end{matrix}}{{normal}\mspace{14mu}{pressure}}}}} & (3)\end{matrix}$

The normal-pressure volume of the nitrogen in the passage is calculatedfrom the overall permeated nitrogen gas amount obtained in the equation(2). The passage volume is the volume of the re-circulation passage fromthe anode 1A through the return passage 5 and a part of the hydrogensupply passage 4 and back to the anode 1A. The normal pressure andvolume of the passage are known values. The anode gas pressure ispresent in both the denominator and the numerator of the equation (3),and is therefore cancelled. Hence the nitrogen partial pressure PNn ofthe anode gas can be calculated from the calculation result of theequation (2).

Having calculated the nitrogen partial pressure PNn of the anode gas inthis manner, the controller 11 reads a temperature TCSAn of the fuelcell stack 1 detected by the temperature sensor 10 in a step S4.

Next, in a step S5, the controller 11 refers to a map having thecharacteristic shown in FIG. 5, which is stored in the internal memory(ROM) in advance, to determine a water vapor partial pressure PWSn ofthe anode gas from the temperature TCSAn of the fuel cell stack 1.

Next, in a step S6, the controller 11 calculates the hydrogen partialpressure PH2n of the anode gas from the nitrogen partial pressure PNndetermined in the step S3 and the water vapor partial pressure PWSndetermined in the step S5 according to the following equation (4).PH2n=anode gas pressure−(PNn+PWSn)  (4)

When the fuel cell stack 1 is operated in a steady state, the anode gaspressure is a constant, known value.

Next, in a step S7, the controller 11 determines variation ΔEDH2n in thegenerated energy EDH2n of the fuel cell stack 1 caused by a reduction inthe hydrogen partial pressure PH2n. For this purpose, a map having thecharacteristic shown in FIG. 6, which defines the relationship betweenthe hydrogen partial pressure PH2n and the generated energy EDH2n, isstored in the internal memory (ROM) of the controller 11 in advance. Thecontroller 11 refers to the map to determine ΔEDH2n from the hydrogenpartial pressure PH2n calculated during execution of the current routineand a hydrogen partial pressure PH2n⁻¹ calculated during execution ofthe previous routine. If the hydrogen partial pressure PH2n hasdecreased, the value of the variation ΔEDH2n in the generated energyEDH2n obtained from the map becomes negative. The variation ΔEDH2ncorresponds to the claimed first energy loss.

Next, in a step S8, the controller 11 calculates variation ΔEDPn in thehydrogen energy EDPn that is lost through purging. To perform thiscalculation, a map having the characteristic shown in FIG. 7 is storedin the internal memory (ROM) of the controller 11 in advance. FIG. 7corresponds to the broken line in FIG. 5. ΔEDPn corresponds to theclaimed second energy loss.

Next, in a step S9, the controller 11 calculates variation ΔEDn in theoverall energy EDn as the sum of the variation ΔEDH2n and the variationΔEDPn.

Next, in a step S10, the controller 11 determines whether or not thevariation ΔEDn in the overall energy EDn is equal to or greater than thevariation ΔEDn−1 that was calculated during execution of the previousroutine.

If the variation ΔEDn is less than ΔEDn−1, this indicates that theoperating condition of the fuel cell stack 1 has not yet reached thepoint A in FIG. 5. If the variation ΔEDn is less than ΔEDn−1, thecontroller 11 ends the routine without rewriting the purge flag fp.

On the other hand, if the variation ΔEDn is equal to or greater thanΔEDn−1, the controller 11 sets the purge flag fp to unity in a step S11,and then ends the routine. By setting the purge flag fp to unity, thecontroller 11 opens the purge valve 8.

If the controller 11 determines that the purge flag fp is at unity inthe aforementioned step S1, or in other words if it is determined thatpurging is underway, the processing of a step S12 is performed.

In the step S12, the controller 11 resets the nitrogen partial pressurePNn of the anode gas to its initial value #PN, and resets the operatingduration tn to zero. The initial value #PN of the nitrogen partialpressure PNn is set at zero or a value approaching zero. These valuesare initialized in order to resume calculation of the energy balancefollowing the end of purging. Following the processing of the step S12,the controller 11 ends the routine.

When the purge valve 8 is opened as a result of the processing of thestep S11, the anode effluent in the return passage 5, which containslarge amounts of impurities such as nitrogen, is purged, and hence thenitrogen content of the anode gas that is supplied to the anode 1Adecreases. When the nitrogen partial pressure of the anode gas hasdecreased sufficiently, the purge valve 8 is closed again.

Various closing conditions may be set as the condition for closing thepurge valve 8, for example closing after remaining open for a fixed timeperiod, closing on the basis of the power generation output of the fuelcell stack 1, and so on. When the purge valve 8 is closed, the purgevalve fp is reset to zero, and the processing of the steps S1-S11 in theroutine in FIG. 4 is resumed from the beginning.

Referring to FIG. 3, by executing the routine described above, thecondition of point A at which the energy balance reaches a minimum canbe detected accurately, and hence the start timing of purging can beoptimized, and energy loss due to purging can be minimized. Moreover, nosensors other than the temperature sensor 11 are required to execute theroutine, and hence control costs can be suppressed.

Next, a second embodiment of this invention will be described withreference to FIGS. 8-13.

Referring to FIG. 8, in addition to the constitution of the firstembodiment, this embodiment comprises an air supply passage 12, an airsupply valve 13, a catalyst 14, and a pump 15. However, the ejector 9has been omitted from this embodiment.

The catalyst 14 is provided at a point on the return passage 5. Thecatalyst 14 functions to remove carbon monoxide (CO) from the anodeeffluent by means of selective oxidation of the carbon monoxidecontained in the anode effluent. The oxidized carbon monoxide thus turnsinto carbon dioxide (CO₂).

The air supply passage 12 is provided to supply air, which serves as theoxidizing agent used by the catalyst 14, from the compressor 2. The airsupply passage 12 is connected to the return passage 5 upstream of thecatalyst 14. The air supply valve 13 serves to adjust the air supplyflow through the air supply passage 12.

The pump 15 is provided on the return passage 5 downstream of thecatalyst 14, and serves in place of the ejector 9 of the firstembodiment to forcibly re-circulate the anode effluent in the returnpassage 5 to the hydrogen supply passage 4.

The opening and closing of the air supply valve 13 and operation of thepump 15 are each controlled by signals from the controller 11.

Carbon monoxide (CO) poisons the catalyst used in the anode 1A, and thuscauses a reduction in the power generation efficiency of the fuel cellstack 1. It is therefore desirable to suppress the carbon monoxidecontained in the anode gas to or below an allowable concentration inorder to maintain the power generation performance of the fuel cellstack 1. For this purpose, the fuel cell power plant according to thisembodiment supplies air to the catalyst 14 from the compressor 2 whenthe carbon monoxide contained in the anode gas reaches or exceeds afixed level, thereby promoting selective oxidation of the carbonmonoxide by the catalyst 14.

The anode effluent that is re-supplied to the anode 1A through thereturn passage 5 contains the following components:

(1) nitrogen from the cathode 1B which permeates the electrolytemembrane 1C to reach the anode 1A,

(2) gas components containing CO other than the hydrogen contained inthe anode gas, and

(3) the air that is supplied to the catalyst 14 in order to remove theCO, and the carbon dioxide (CO₂) that is produced as a result of theselective oxidation reaction of the catalyst 14.

In this embodiment, the amount of nitrogen in the air supplied to thecatalyst 14 and the amount of carbon dioxide produced in the catalyst 14are taken into consideration when calculating the hydrogen partialpressure PH2n of the anode gas.

Next, referring to the flowcharts in FIGS. 9-11, various routinesexecuted by the controller 11 to perform the above control will bedescribed.

FIG. 9 is a purging start determination routine, and corresponds to theroutine in FIG. 4 of the first embodiment.

The processing of steps S21-S23 is identical to that of the steps S1-S3in the routine in FIG. 4, and hence description thereof has beenomitted.

It should be noted, however, that the calculation method of the step S3can only be applied to the calculation of the nitrogen partial pressurePNn in the step S23 when a CO removal flag fdco to be described below isat zero. When the CO removal flag fdco is at unity, the controller 11applies a newest value of the nitrogen partial pressure PNn, which iscalculated in a step S55 of the routine shown in FIG. 11 to be describedbelow, as the nitrogen partial pressure.

In a step S24, the controller 11 calculates a partial pressure PCO2n ofthe carbon dioxide contained in the anode gas. Here, a carbon dioxideamount MCO2n in the anode gas is used to calculate the carbon dioxidepartial pressure PCO2n by means of a similar method to the equation (3)for calculating the nitrogen partial pressure which was learned in thestep S3 (S23). The carbon dioxide amount MCO2n in the anode gas is setas the newest value of the carbon monoxide amount in the anode gas whichis calculated in a step S53 or a step S57 of FIG. 11, to be describedbelow.

Next, in a step S25, the controller 11 reads a temperature TCSAn of thefuel cell stack 1 detected by the temperature sensor 10 similarly to thestep S5.

Next, in a step S26, the controller 11 detects the water vapor partialpressure PWSn using the same method as that of the step S6.

Next, in a step S27, the controller 11 calculates the hydrogen partialpressure PH2n according to the following equation (5).PH2n=anode gas pressure·(PNn+PWSn+PCO2n)  (5)

The difference between the equation (5) and the equation (4) is that inthe equation (5), the carbon dioxide partial pressure PCO2n is takeninto consideration. The anode gas also contains carbon monoxide (CO),but most of the carbon monoxide is converted into carbon dioxide (CO₂)due to the selective oxidation action of the catalyst 14, and hence theCO concentration in the anode gas is negligible compared to thenitrogen, water vapor, and carbon dioxide concentrations. Accordingly,the carbon monoxide (CO) partial pressure is ignored when calculatingthe hydrogen partial pressure PH2n.

The processing of steps S28-S32 is identical to that of the steps S7-S11in the first embodiment.

When the controller 11 determines in the step S21 that the purge flag fpis at unity, the processing of a step S33 is performed. In the step S33,in addition to performing identical reset processing to that of the stepS12 in the first embodiment, the CO removal flag fdco is reset to zero.

Next, referring to FIG. 10, a carbon monoxide removal determinationroutine executed by the controller 11 will be described. The controller11 implements this routine repeatedly at a fixed interval Δt during anoperation of the fuel cell power plant. The fixed interval Δt is set at100 milliseconds.

In a first step S41, the controller 11 determines whether or not thepurge flag fp is at unity. If the purge flag fp is at unity, or in otherwords if anode effluent purging is underway, the controller 11immediately ends the routine.

When the purge flag fp is not at unity, or in other words when purgingis not underway, the controller 11 calculates a CO concentration DCOn inthe anode gas in a step S42.

For this purpose, the controller 11 first refers to a map having thecharacteristic shown in FIG. 12, which is stored in the internal memory(ROM) in advance, to determine a hydrogen supply flow rate ΔMH2 from thehydrogen tank 3 to the hydrogen supply passage 12 on the basis of theload of the fuel cell stack 1.

Since the hydrogen supplied from the hydrogen tank 3 contains a fixedamount of carbon monoxide (CO), a CO amount MCOn that is mixed into theanode gas can be calculated from the hydrogen supply flow rate ΔMH2according to the following equation (6).MCOn=MCOn⁻¹+A·ΔMH2  (6)

where, MCOn⁻¹=MCOn calculated during execution of the previous routine,and

-   -   A=the CO concentration in the hydrogen supplied during the time        period Δt=a constant.

The CO concentration DCOn in the anode gas is determined by dividing theCO amount MCOn mixed into the anode gas by the anode gas mass.

It should be noted that the above description of the calculation of theCO concentration DCOn in the anode gas relates to a calculation methodperformed when carbon monoxide removal is not underway in the routine inFIG. 11 to be described below. When carbon monoxide removal is underwayin the routine in FIG. 11, the controller 11 calculates the COconcentration DCOn in the anode gas using the newest CO amount MCOncalculated in a step S54 in FIG. 11.

The anode gas contains nitrogen, carbon dioxide, water vapor, andhydrogen as well as carbon monoxide. The partial pressures of thenitrogen, water vapor, and hydrogen in the anode gas have already beencalculated in the steps S23, S26, and S27 in FIG. 9.

The controller 11 determines the mass of nitrogen, water vapor, andhydrogen in the anode gas from the newest values of each of thesepartial pressures. To determine the mass of the carbon dioxide containedin the anode gas, the controller 11 uses the newest value of the CO₂amount MCO2n which is calculated in the step S53 of the routine in FIG.11 to be described below.

The controller 11 calculates the anode gas mass by adding the CO amountMCOn to the total mass of the nitrogen, carbon dioxide, water vapor, andhydrogen masses determined in the manner described above. The controllerdetermines the CO concentration DCOn by determining the CO amount MCOnthat is mixed into the anode gas according to the equation (6) anddividing the CO amount MCOn by the anode gas mass.

Next, in a step S43, the controller 11 determines whether or not the COconcentration DCOn exceeds a predetermined concentration #SHDCO.

When the CO concentration DCOn exceeds the predetermined concentration#SHDCO, the controller 11 sets the CO removal flag fdco to unity in astep S45. If the CO concentration DCOn does not exceed the predeterminedconcentration #SHDCO, the controller 11 resets the CO removal flag fdcoto zero in a step S44.

The predetermined concentration #SHDCO is set with a sufficient safetymargin using a concentration at which the performance of the anode 1Adeteriorates due to carbon monoxide poisoning as a reference.

Following the processing of the step S44 or the step S45, the controller11 ends the routine.

Next, referring to FIG. 11, a carbon monoxide removal routine, which isexecuted by the controller 11 on the basis of the CO removal flag fdco,will be described. The controller 11 implements this routine repeatedlyat a fixed interval Δt during an operation of the fuel cell power plant.The fixed interval Δt is set at 100 milliseconds.

In a first step S50, the controller 11 determines whether or not thepurge flag fp is at unity. If the purge flag fp is at unity, purging isunderway, and in this case the routine is ended immediately.

If the purge flag fp is not at unity, a determination is made in a stepS51 as to whether or not the CO removal flag fdco is at unity. If the COremoval flag fdco is at unity, the processing of steps S52-S55 isperformed to remove carbon monoxide from the anode gas.

If the CO removal flag fdco is not at unity, then carbon monoxideremoval is not necessary. In this case, the controller 11 operates thepump 15 at its normal rotation speed in a step S56.

Next, in a step S57, the CO₂ amount MCO2n in the anode gas is set to beequal to the value thereof MCO2n⁻¹ obtained during execution of theprevious routine. Following the processing of the step S57, thecontroller 11 ends the routine.

In the steps S52-S55, the controller 11 removes carbon monoxide from theanode gas in the following manner.

First, in the step S52, the controller 11 reduces the rotation speed ofthe pump 15.

Next, in the step S53, the controller 11 calculates the CO₂ amount MCO2nin the anode gas according to the following equation (7).MCO2n=MCO2n⁻¹+δ·MCO2  (7)

where, δMCO2=the amount of increase in the CO₂ amount MCO2n, and

-   -   MCO2n⁻¹=MCO2n calculated during execution of the previous        routine.

MCO2 is a fixed value when the opening of the air supply valve 13 isfixed. In cases where the opening of the air supply valve 13 is varied,the controller 11 refers to a map, which is created in advance to definethe relationship between the opening of the air supply valve 13 and theamount of CO₂ generated by the catalyst 14 per unit time, to determinethe amount of generated CO₂ from the opening of the air supply valve 13,and then sets the amount of generated CO₂ as the amount of increase inthe CO₂ amount MCO2n.

Next, in the step S54, the controller 11 corrects the CO amount MCOn inthe anode gas using the following equation (8).MCOn=MCOn⁻¹−δ·MCO  (8)

where, MCOn⁻¹=the newest value of the CO₂ amount MCO2n calculated in thestep S42 of FIG. 10, and

-   -   δMCO=the amount of decrease in the CO amount accompanying the        generation of CO₂.

δMCO is calculated from the amount of increase δMCO2 in the CO₂ amount.The CO amount MCOn after correction is used in the manner describedabove to calculate the CO concentration DCOn in the anode gas in thestep S42 when the routine of FIG. 10 is next executed.

Next, in the step S55, the controller 11 recalculates the nitrogenpartial pressure PNn of the anode gas that was calculated in the stepS23 of the routine in FIG. 9.

This recalculation is performed due to the fact that the nitrogenpartial pressure in the anode gas is increased by the nitrogen containedin the air that is supplied to the return passage 5 to perform COremoval processing. When the fuel cell stack 1 is operating normally,the air supply pressure of the compressor 2 is constant, and hence theflow rate of the air that is supplied to the return passage 5 can bedetermined from the opening of the air supply valve 13. Accordingly, theflow rate of the nitrogen that is supplied to the return passage 5 canalso be calculated from the opening of the air supply valve 13. Thecontroller 11 converts the nitrogen flow into a nitrogen increase amountper routine execution interval Δt, adds the value thereof to thepermeated nitrogen gas amount of the equation (1), and thus determinesthe nitrogen increase amount per Δt. The nitrogen increase amount istime-integrated in a similar fashion to the equation (2), whereupon theequation (3) is applied to the obtained permeated nitrogen gas amount toobtain the nitrogen partial pressure PNn of the anode gas. The nitrogenpartial pressure PNn of the anode gas thus recalculated is used in thestep S27 of the next execution of the routine in FIG. 9 as describedabove.

Following the processing of the step S55, the controller ends theroutine.

According to this embodiment, purging is performed at the mostappropriate point on the basis of the energy balance, similarly to thefirst embodiment. Further, carbon monoxide (CO) removal is performed onthe basis of the CO concentration DCOn in the anode gas. Hencereductions in the power generation efficiency of the fuel cell stack 1due to carbon monoxide poisoning of the anode 1A can also be prevented.

As a result of carbon monoxide accumulation, the anode effluent that isre-circulated into the hydrogen supply passage 4 from the return passage5 has a higher CO concentration than the anode gas which is constitutedby anode effluent and hydrogen supplied from the hydrogen tank 3. Inthis embodiment, the catalyst 14 which selectively oxidizes carbonmonoxide is provided in the return passage 5 having a high COconcentration, and hence carbon monoxide can be oxidized efficiently.

Referring to FIG. 13, the selective oxidation catalyst 14 exhibits ahigher conversion rate, or in other words a better CO oxidationperformance, as the spatial velocity SV of the subject gas decreases. Byproviding the catalyst 14 in the return-passage 5 having a low spatialvelocity SV, the catalyst 14 exhibits a good CO oxidation performance,and as a result, the catalyst 14 can be reduced in size.

In the step S52 of FIG. 11, the rotation speed of the pump 15 is reducedat the time of carbon monoxide removal. This is in order to reduce theanode effluent re-circulation rate temporarily such that the flow rateof anode effluent flowing out of the anode 1A into the return passage 5is reduced and the CO concentration is increased. As a result, mixing ofthe anode effluent with the air that is supplied to the return passage 5from the air supply passage 12 is promoted, and thus the downstreamcatalyst 14 can perform carbon monoxide oxidation using little air. Inso doing, the reduction in hydrogen partial pressure which accompaniesthe introduction of air can be suppressed.

In this embodiment, the carbon dioxide partial pressure PCO2n is takeninto consideration when calculating the hydrogen partial pressure PH2nin the routine in FIG. 9. Moreover, in the step S55 in FIG. 11, thenitrogen partial pressure PNn is recalculated taking into considerationthe air that is supplied for carbon monoxide removal. The effect of theair supplied for carbon monoxide removal and the effect caused by carbonmonoxide oxidation are thus reflected in the calculation of the hydrogenpartial pressure PH2n, and as a result, carbon monoxide removal can beperformed without impairing the precision with which the purging starttiming is determined.

In this embodiment, the amount of carbon monoxide contained in the anodegas is calculated on the basis of the amount of hydrogen supplied fromthe hydrogen tank 3, and carbon monoxide removal is performed when theCO concentration DCOn that is calculated from the carbon monoxide amountexceeds the predetermined concentration #SHDCO. As a result, the timingof carbon monoxide removal can be determined accurately.

Next, referring to FIG. 14, a third embodiment of this invention will bedescribed.

In this embodiment, a recording device 16 using an integrated circuit(IC) is annexed to the hydrogen tank 3 of the fuel cell power plantaccording to the second embodiment. The CO concentration of the hydrogenthat is stored in the hydrogen tank 3 is recorded on the recordingdevice 16. The CO concentration data are extracted when the hydrogentank 3 is refilled with hydrogen by means of data communication betweenthe recording device 16 and a refill station which refills the hydrogentank 3 with hydrogen.

In cases where the hydrogen for refilling the hydrogen tank 3 containsimpurities caused by other inert gases, the recording device 16preferably records further impurity concentration data.

The recording device 16 and controller 11 are connected by a signalcircuit, and the CO concentration and impurity concentration areprovided to the controller 11 as signals. Similarly to the secondembodiment, the controller 11 determines the purging start ting andperforms carbon monoxide removal by executing the routines in FIGS.9-11. In the step S42 in FIG. 10, the equation (6) is used to calculatethe CO amount MCOn in the anode gas. At this time, the controller 11 candetermine the constant A using the CO concentration and impurityconcentration read from the recording device 16.

According to this embodiment, the CO concentration of the hydrogensupplied from the hydrogen tank 3 can be grasped accurately without theuse of a gas concentration sensor, and as a result, variation in the COconcentration DCOn in the anode gas during an operation of the fuel cellstack 1 can be calculated accurately.

The contents of Tokugan 2003-147996, with a filing date of May 26, 2003in Japan, are hereby incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art,within the scope of the claims.

For example, in each of the embodiments described above, this inventionis applied to a fuel cell power plant which generates power usinghydrogen from the hydrogen tank 3. However, this invention is alsoapplicable to a fuel cell power plant which extracts hydrogen byreforming hydrocarbon fuel.

INDUSTRIAL FIELD OF APPLICATION

As described above, in this invention, the start timing of purging isdetermined on the basis of the energy balance during power generationperformed by a fuel cell stack while anode effluent is re-circulatedinto anode gas, and hence the fuel cell stack can be operated withfavorable power generation efficiency at all times. Accordingly, thisinvention exhibits particularly favorable effects when applied to a fuelcell power plant for driving a vehicle, in which limitations on thehydrogen storage capacity and fuel cell stack disposal space are severe.

The invention claimed is:
 1. An anode effluent control method for a fuel cell power plant comprising a fuel cell stack which performs power generation using anode gas having hydrogen as a main component, the anode gas being discharged from the fuel cell stack as anode effluent following a power generation reaction, a return passage which re-circulates the anode effluent into the anode gas, and a purge valve which discharges the anode effluent in the return passage to the outside of the passage, the control method comprising: calculating a first energy loss caused by an increase in a non-hydrogen component in the anode gas while the purge valve is closed; calculating a second energy loss which corresponds to an amount of hydrogen lost from the anode gas when the purge valve is opened; maintaining the purge valve in a closed state when the second energy loss is larger than the first energy loss; and opening the purge valve when the second energy loss equals or falls below the first energy loss.
 2. The anode effluent control method as defined in claim 1, wherein the non-hydrogen component includes nitrogen and water vapor.
 3. The anode effluent control method as defined in claim 2, wherein the control method further comprises: calculating a nitrogen partial pressure of the anode gas in accordance with a duration of the closed state of the purge valve; determining a temperature of the fuel cell stack; calculating a water vapor partial pressure of the anode gas on the basis of the temperature of the fuel cell stack; calculating a hydrogen partial pressure of the anode gas by subtracting the nitrogen partial pressure and the water vapor partial pressure from an anode gas pressure; and calculating the first energy loss on the basis of variation in the hydrogen partial pressure.
 4. The anode effluent control method as defined in claim 3, wherein the fuel cell stack comprises an anode which is exposed to the anode gas, a cathode, and an electrolyte membrane disposed between the anode and the cathode, the fuel cell power plant further comprises an air supply device which supplies air to the cathode, and the control method further comprises calculating the nitrogen partial pressure of the anode gas on the basis of an amount of nitrogen in the anode gas which increases as nitrogen in the air permeates the electrolyte membrane from the cathode so as to reach the anode.
 5. The anode effluent control method as defined in claim 3, wherein the fuel cell power plant further comprises an anode gas passage which supplies the anode gas to the fuel cell stack, a hydrogen supply device which supplies hydrogen to the anode gas passage, a catalyst which oxidizes carbon monoxide in the anode effluent in the return passage, and an air supply device which supplies air for oxidizing the carbon monoxide to the return passage, and the control method further comprises calculating an accumulated amount of the carbon monoxide in the anode gas that was contained in the hydrogen supplied to the anode gas passage from the hydrogen supply device, comparing the accumulated amount to a predetermined value, and supplying air to the return passage from the air supply device when the accumulated amount is larger than the predetermined value.
 6. The anode effluent control method as defined in claim 5, wherein the control method further comprises preventing the air supply device from supplying air to the return passage when the purge valve is open.
 7. The anode effluent control method as defined in claim 5, wherein the fuel cell power plant further comprises a pump for pressurizing the anode effluent in the return passage so as to introduce the anode effluent into the anode gas passage, and the control method further comprises reducing the rotation speed of the pump when air is supplied to the return passage from the air supply device.
 8. The anode effluent control method as defined in claim 5, wherein the control method further comprises calculating a partial pressure of carbon dioxide that is mixed into the anode gas as a result of a carbon monoxide oxidation operation performed by the catalyst, correcting the nitrogen partial pressure of the anode gas on the basis of an amount of air supplied to the return passage, and calculating the hydrogen partial pressure by subtracting the water vapor partial pressure, the carbon dioxide partial pressure, and a corrected nitrogen partial pressure from the anode gas pressure.
 9. The anode effluent control method as defined in claim 5, wherein the fuel cell power plant further comprises a recording device which pre-records a carbon monoxide content of the hydrogen that is supplied by the hydrogen supply device, and the control method further comprises calculating the accumulated amount of carbon monoxide in the anode gas on the basis of the carbon monoxide content recorded in the recording device.
 10. The anode effluent control method as defined in claim 3, wherein the control method further comprises calculating the nitrogen partial pressure of the anode gas as a value which increases as a duration of the closed state of the purge valve lengthens.
 11. The anode effluent control method as defined in claim 3, wherein the control method further comprises calculating the water vapor partial pressure of the anode gas as a value which increases as a temperature of the fuel cell stack rises.
 12. The anode effluent control method as defined in claim 1, wherein the control method further comprises calculating the second energy loss as a value which decreases in accordance with a duration of the closed state of the purge valve.
 13. An anode effluent control device for a fuel cell stack which performs power generation using anode gas having hydrogen as a main component, the anode gas being discharged from the fuel cell stack as anode effluent following a power generation reaction, the device comprising: a return passage which re-circulates the anode effluent into the anode gas; a purge valve which discharges the anode effluent in the return passage to the outside of the passage; and a programmable controller programmed to: calculate a first energy loss caused by an increase in a non-hydrogen component in the anode gas while the purge valve is closed; calculate a second energy loss which corresponds to an amount of hydrogen lost from the anode gas when the purge valve is opened; maintain the purge valve in a closed state when the second energy loss is larger than the first energy loss; and open the purge valve when the second energy loss equals or falls below the first energy loss.
 14. The anode effluent control device as defined in claim 13, wherein the non-hydrogen component includes nitrogen and water vapor.
 15. The anode effluent control device as defined in claim 14, wherein the controller is further programmed to: calculate a nitrogen partial pressure of the anode gas in accordance with a duration of the closed state of the purge valve; determine a temperature of the fuel cell stack; calculate a water vapor partial pressure of the anode gas on the basis of the temperature of the fuel cell stack; calculate a hydrogen partial pressure of the anode gas by subtracting the nitrogen partial pressure and the water vapor partial pressure from an anode gas pressure; and calculate the first energy loss on the basis of variation in the hydrogen partial pressure.
 16. The anode effluent control device as defined in claim 15, wherein the fuel cell stack comprises an anode which is exposed to the anode gas, a cathode, and an electrolyte membrane disposed between the anode and the cathode, the device further comprises an air supply device which supplies air to the cathode, and the controller is further programmed to calculate the nitrogen partial pressure of the anode gas on the basis of an amount of nitrogen in the anode gas which increases as nitrogen in the air permeates the electrolyte membrane from the cathode so as to reach the anode.
 17. The anode effluent control device as defined in claim 15, wherein the device further comprises an anode gas passage which supplies the anode gas to the fuel cell stack, a hydrogen supply device which supplies hydrogen to the anode gas passage, a catalyst which oxidizes carbon monoxide in the anode effluent in the return passage, and an air supply device which supplies air for oxidizing the carbon monoxide to the return passage, and the controller is further programmed to calculate an accumulated amount of the carbon monoxide in the anode gas that was contained in the hydrogen supplied to the anode gas passage from the hydrogen supply device, compare the accumulated amount to a predetermined value, and supply air to the return passage from the air supply device when the accumulated amount is larger than the predetermined value.
 18. The anode effluent control device as defined in claim 17, wherein the controller is further programmed to prevent the air supply device from supplying air to the return passage when the purge valve is open.
 19. The anode effluent control device as defined in claim 17, wherein the device further comprises a pump for pressurizing the anode effluent in the return passage so as to introduce the anode effluent into the anode gas passage, and the controller is further programmed to reduce the rotation speed of the pump when air is supplied to the return passage from the air supply device.
 20. The anode effluent control device as defined in claim 17, wherein the controller is further programmed to calculate a partial pressure of carbon dioxide that is mixed into the anode gas as a result of a carbon monoxide oxidation operation performed by the catalyst, correct the nitrogen partial pressure of the anode gas on the basis of an amount of air supplied to the return passage, and calculate the hydrogen partial pressure by subtracting the water vapor partial pressure, the carbon dioxide partial pressure, and a corrected nitrogen partial pressure from the anode gas pressure.
 21. The anode effluent control device as defined in claim 17, wherein the device further comprises a recording device which pre-records a carbon monoxide content of the hydrogen that is supplied by the hydrogen supply device, and the controller is further programmed to calculate the accumulated amount of carbon monoxide in the anode gas on the basis of the carbon monoxide content recorded in the recording device.
 22. The anode effluent control device as defined in claim 15, wherein the controller is further programmed to calculate the nitrogen partial pressure of the anode gas as a value which increases as a duration of the closed state of the purge valve lengthens.
 23. The anode effluent control device as defined in claim 15, wherein the controller is further programmed to calculate the water vapor partial pressure of the anode gas as a value which increases as a temperature of the fuel cell stack rises.
 24. The anode effluent control device as defined in claim 13, wherein the controller is further programmed to calculate the second energy loss as a value which decreases in accordance with a duration of the closed state of the purge valve.
 25. An anode effluent control device for a fuel cell stack which performs power generation using anode gas having hydrogen as a main component, the anode gas being discharged from the fuel cell stack as anode effluent following a power generation reaction, the device comprising: a return passage which re-circulates the anode effluent into the anode gas; a purge valve which discharges the anode effluent in the return passage to the outside of the passage; means for calculating a first energy loss caused by an increase in a non-hydrogen component in the anode gas while the purge valve is closed; means for calculating a second energy loss which corresponds to an amount of hydrogen lost from the anode gas when the purge valve is opened; means for maintaining the purge valve in a closed state when the second energy loss is larger than the first energy loss; and means for opening the purge valve when the second energy loss equals or falls below the first energy loss. 